Contact between catalyst particles and gaseous reactants routinely occurs in reaction vessels for production of chemicals, the conversion of hydrocarbons, or the rejuvenation of catalyst. Typically process arrangements retain the catalyst in a fixed bed, as a semicontinuously moving bed or in a fluidized state. An increasing number of reaction arrangements are practiced or proposed for the fluidized transport and contacting of particulate catalyst with gas streams. Such processes include catalytic cracking of hydrocarbons, dehydrogenation processes and olefin production from methanol.
In a fluidized system catalyst particles are transported like a fluid by passing gas or vapor through the particles at a sufficient velocity to eliminate friction between the catalyst particles and to produce a desired regime of fluid behavior with the solid particles. Fluidized catalyst systems are most useful for processes that have rapid catalyst deactivation. Most of these processes rapidly lay coke down on the catalyst as a by-product of the reaction. Coke deactivates the catalyst. The fluidized transport provides the necessary high circulation of solids between a reaction zone that generates the coke and a regeneration zone that removes coke from the catalyst. High catalyst circulation, also referred to as catalyst mass flux, is a key to controlling the accumulation of coke on the catalyst. Conventional regeneration operations oxidatively combust coke from the surface of the catalyst to reduce the coke levels before returning the catalyst to the reaction zone.
The fluidized catalytic cracking of hydrocarbons is the most familiar example of a fluidized catalytic reaction system. In the FCC process large hydrocarbon molecules associated with a heavy hydrocarbon feed are cracked thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as products, primarily gasoline, and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed. The FCC process is carried out by contacting the starting material--whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons--with a catalyst made up of a finely divided or particulate solid material. Contact of the oil with hot fluidized catalyst catalyzes the cracking reaction. During the cracking reaction, coke deposits on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a reaction zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by countercurrent contact with steam or another stripping medium. The stripping medium displaces hydrocarbon vapor from the interstitial space between catalyst particles and from the internal pore volume of the catalyst particles. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected in the regeneration zone for return to the reaction zone.
Oxidizing the coke from the catalyst surface releases a large amount of heat; a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then is circulated again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from the regeneration zone to reaction zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. As a result the rate of catalyst circulation through the regeneration zone varies throughout the routine operation of the process.
Separate and distinct separation systems are used to separate gases from particles on both the reaction and regeneration sides of the process. Each system will use a two stage separation with a first initial disengagement stage that separates most of the particles from the gas and a secondary separation stage that further reduces the particulate levels in the gas stream.
After particulate removal the cracked hydrocarbons of the FCC reaction are recovered in vapor form and transferred to product recovery facilities. These facilities normally comprise a main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked fractions which usually include bottom materials, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams. The heaviest fraction of the separated hydrocarbon vapors will contain any residual particulate material that enters with the incoming vapors. Thus, particulate material that is not recovered by the separation systems of the reactor may still be readily recovered downstream in the heaviest hydrocarbon fractions.
Following separation of particulate material in the regeneration zone, flue gases undergo appropriate treatment for removal of pollutants such as sulfur and nitrogen compounds and particulate material and are then discharged to the atmosphere. Therefore, recovering as much particulate material as possible from the flue gas is especially important on the regenerator side of the process to avoid discharge of particulate material to the atmosphere and to reduce downstream treatment costs for the flue gas. The minimization of catalyst particle carryover has become of increasing concern due to environmental restrictions on the discharge of particulate materials. Consequently, all commercially practiced separation systems for regenerators rely exclusively on a two stage cyclone system for removing the fine particles of entrained catalyst from the gases before the gases exit the system. As a result a firmly entrenched practice has evolved wherein two stages of cyclone separators are used to minimize any carryover of catalyst particles with the flue gas exiting the regeneration vessel.
Different consideration and criteria have influenced the approach to separating catalyst from gas streams on the reactor and the regenerator sides of the process. The reactor vapors are not discharged to the atmosphere; as a result, higher catalyst loadings do not generate air pollution concerns. Since contact time between catalyst and reactants can have profound effects on product quality, quick separation of catalyst from reaction vapors is sought. On the regeneration side, contact time between flue gases and catalyst is less critical and fast separation has not been sought. Consistent high efficiency separation is the primary goal on the regeneration side of the process.
For many years the reactor and regenerator side of the process operated with a large open vessel that served as a disengaging chamber for an initial separation of the catalyst from the product vapors. The large volume of the vessel provided an initial gravitational or settling type separation of particles from the gases. It was commonplace for the gravitational separation to occur in a dilute phase above a large dense phase catalyst bed. (The terms "dense phase" and "dilute phase" catalysts as used in this application are meant to refer to the density of the catalyst in a particular zone. The term "dilute phase" generally refers to a catalyst density of less than 20 lbs/ft.sup.3 and the term "dense phase" refers to catalyst densities above 30 lbs/ft.sup.3. Catalyst densities in the range of 20-30 lbs/ft.sup.3 can be considered either dense or dilute, depending on the density of the catalyst in adjacent zones or regions.) Rising gases from a large open vessel go through a further stage or stages of inertial separation, most often in one or more stages of cyclone separator. The diameter of the large vessel was sized to maintain a superficial gas velocity upward through the regeneration vessel at a rate selected to minimize the entrainment of catalyst particles above the surface of the bed and ultimately into the cyclone separators.
In an effort to reduce residence time, the reactor side of the process replaced the initial stage of gravitational separation with a more contained inertial separation that reduces contact time between the catalyst and hydrocarbon vapors. Examples of such contained inertial systems are direct connected cyclones (U.S. Pat. No. 4,737,346), enclosed ballistic separation (U.S. Pat. No. 4,792,437) and a tangential entry separator (U.S. Pat. 4,482,451). In addition to providing the desired reduction in dilute phase residence time of the hydrocarbon vapors, the replacement of the initial gravitational separation with inertial separation provided a more compact and cost effective design for the reactor side of the process.
Despite changes to the reactor separation system, the early and current regeneration process arrangements continue to use relatively large regeneration vessels as a settling zone for an initial division between fine catalyst particles and flue gases that then traditionally enter two downstream stages of cyclone separators. The large disengagement vessel provides consistent disengagement despite changes in catalyst circulation rate or pressure surges in the regeneration zone. The consistent, initial separation of the catalyst provided by the gravitation or settling disengagement of catalyst from flue gases prevents overloading of the cyclones and maintains the high separation efficiency desired to minimize entrainment of catalyst beyond the regeneration zone cyclones. Providing the large volume disengaging vessel and dual stages of cyclones on the regeneration side of the process affects the design of the regeneration vessel and imposes additional costs on the construction of regeneration vessels and the associated equipment. Proposed regeneration arrangements that have eliminated the large disengaging vessel still regularly employ at least dual stages of cyclones to provide the required separation of efficiency and do not address the potential for cyclone overload and temporary carryover of catalyst from the regeneration zone.
The mechanics of the regeneration process also reinforced the perceived need for a dilute phase regenerator. As the oxygen-containing gas contacts the coke on the catalyst particles at high temperature, reaction of the coke with oxygen forms CO as the principal reaction product and regenerates catalyst particles. Along with the conversion of coke to CO, a secondary reaction of converting CO to CO.sub.2 also occurs in the regeneration of the catalyst particles. Both reactions are highly exothermic. Catalyst densities in the large disengaging vessel are typically 1 lb/ft.sup.3 or less. Operators of the early dense phase regenerators were concerned that combustion of CO to CO.sub.2 in the dilute phase above the catalyst bed of the regeneration vessel would generate high amounts of heat without the presence of a sufficient heat sink, i.e., catalyst, to prevent temperature excursions which could exceed 1500.degree. F. Accordingly, regeneration vessels operated with limited air or oxygen addition to the catalyst bed to prevent the breakthrough of oxygen above the bed into the dilute phase of the regeneration vessel. Transport risers that operated with excess oxygen and a relatively dense catalyst phase were added above the dense bed to complete combustion of CO to CO.sub.2 in regeneration zones. The transport zone operated with catalyst densities in the range of from 5 to 10 lb/ft.sup.3 and superficial gas velocities of about 10-25 ft/sec.
In addition to the reactions and catalyst separation, fluidized systems must also provide the necessary hydraulics for the transport of the particulate material between the different zones. Elevation of particulate material to a particular zone for purposes of catalyst transport to a subadjacent zone can be accomplished by a conduit dedicated solely for a lift purpose, but is more efficiently conducted when the lift step provides an additional function. In regenerator arrangements where regenerated catalyst is transferred to an elevated location of the reactor, the lifting of catalyst is usually taking place relatively independently from the regeneration of the catalyst by coke oxidation. Coke oxidation is primarily carried out in a dense phase where long residence time contacting between the catalyst particles and oxygen can take place. Lifting of the catalyst is usually occurring after dense phase oxidation of coke from the catalyst with minimal initial oxidation of coke in a dilute phase. Using a more dense phase combustion zone for combined transport and regeneration of catalyst has more susceptibility to variations in catalyst loading on the separation system; therefore, transport conduits for regenerated catalyst have generally been limited to relatively low densities that inhibit the essentially complete removal of coke from catalyst for full regeneration and are used with multiple stages of cyclones.
It is an object of this invention to provide an initial separation system for a regeneration process which operates with a high separation efficiency and which can accommodate temporary catalyst loadings.
It is a further object of this invention to provide an initial separator of regenerated catalyst and gases that is compatible for use with a dense phase lift conduit for transport and simultaneous combustion of coke from catalysts.
It is a yet further object of this invention to operate a large volume combustor riser in a regeneration process with a single stage of cyclones and to provide catalyst lift for simplifying hydraulics.
It is a further object of this invention to operate a combustion riser such that the discharge of catalyst from the riser permits the use of single stage cyclones and has suitable flexibility in the operation to accommodate changes in density without overloading the cyclones.